1. Field of the Invention
The present invention relates to a thin-film nonlinear diode for use in a large-area, large-capacity active-type liquid crystal display (LCD) applicable to wall-hung televisions, flat displays for computer terminal units and the like.
2. Description of the Related Art
As thin panel displays used, for example, in wall-hung color televisions, there have been developed several types of active matrix liquid crystal displays in which active elements are arrayed on a glass substrate on one side as a switch for each pixel. With this active matrix system, it becomes possible to provide a large capacity LCD without lowering display quality. Thin film nonlinear diode elements, which include a nonlinear resistance layer having conductive electrodes on both opposite sides and which are simpler in structure than thin-film transistors, are usable as active elements and are expected to reduce manufacturing cost.
Heretofore, tantalum oxide has been employed for the nonlinear resistance layer. However, the dielectric constant of this material is as large as 25. Moreover, the film thickness of this nonlinear resistance layer has to be less than 60 nm to make current flow at approximately 30 V. As a result, the stray capacitance of the nonlinear resistance element per unit area can be large. To drive the thin-film nonlinear diode element, the stray capacitance of the thin-film nonlinear diode element has to be sufficiently smaller than that of the liquid crystal. To satisfy this requirement, the area of the thin-film nonlinear diode element of tantalum oxide must be reduced to approximately 20 .mu.m.sup.2 or smaller It is extremely difficult to provide such microminiature elements over 14-inches diagonal area at the present level of technology.
Japanese Patent Laid-Open No. 61-260219 (issued on Nov. 18, 1986), for instance, discloses the use of a silicon oxide or silicon nitride film having a dielectric constant of 5 to 7 as a nonlinear resistance element in order to solve the capacitance problems. According to this disclosure, the silicon oxide or nitride film can have a nonlinear coefficient equal to or larger than 8 even though its thickness exceeds 100 nm, whereby a nonlinear resistance element having an area of over 100 .mu.m.sup.2 is usable.
However, if a nonlinear resistance element of silicon oxide or silicon nitride is used, a voltage greater than 20 V is required to produce current large enough to charge the liquid crystal, because the resistance of the silicon oxide or nitride film is still large. In this improvement, there arises another problem that a design of such high voltage drive IC for the LCD is difficult in practice.